1. Field of the Invention
The present invention relates to a semiconductor light detecting device, and specifically to a semiconductor light detecting device for monitoring a laser output of a semiconductor laser which is mounted on a module having the semiconductor laser on it.
A semiconductor laser varies in its optical output under the influence of heat generation of the device itself, the ambient temperature around it, and the like even if its driving current is kept constant.
As a method for keeping an output of a semiconductor laser constant, it is known to control a driving current of the semiconductor laser through an automatic power control (APC) circuit so that output current of a light detecting device may be kept constant which is disposed behind the semiconductor laser as a monitoring light detecting device and is irradiated with the light outputted from the rear part of the semiconductor laser. The method of keeping the output of a semiconductor laser in such a manner as this is described in Japanese First Publication Tokkaihei 5-29712, for example.
The APC circuit, for example, is made to monitor the output of the semiconductor laser by measuring the average output current of the light detecting device in a period and feed it back into the laser diode driving circuit.
A pin photodiode as shown in FIGS. 1A and 1B, for example, is used as the light detecting device for monitoring. FIG. 1A is a plan view of the photodiode and FIG. 1B is a cross-sectional view of it.
In the Figures, an n.sup.- -type InGaAs layer 102 and an n.sup.- -type InP layer 103 are formed on an n.sup.+ -type InP substrate 101 in order, and a p.sup.+ -type diffusion layer 104 which acts as an active area is formed in the n.sup.- -type InP layer 103. The n.sup.- -type InP layer 103 is covered with a passivation film 105 of silicon nitride, and an opening 106 is formed in the passivation film 105 on the circumference part of the n.sup.+ -type diffusion layer 104, and an p-electrode 107 formed on the passivation layer 105 is connected with the p.sup.+ -type diffusion layer 104 through the opening 106. An n-electrode 108 is formed under the n.sup.+ -type InP substrate 101.
Light outputted from the rear part of the semiconductor laser is irradiated almost all over the face of the light detecting device for monitoring which is shown in FIGS. 1A and 1B.
Electrons and holes are generated inside the light detecting device by irradiation of the light. The electrons are moved to an n-type domain (the n.sup.+ -type InP substrate 101) through an electric field generated by a diffusion potential of the pn junction, while the holes flow into a p type domain (the p.sup.+ -type impurity diffusion layer 104).
Electrons and holes generated in a region distant from the depletion layer which exists in the vicinity of the interface between the p.sup.+ -type diffusion layer 104 and both of the n.sup.- -type InGaAs layer 102 and the n.sup.- -type InP layer 103 reach the depletion layers through diffusion, but it takes an extra time for these electrons and holes to reach there, so they become slow components in respect of response speed.
Since the slow components can scarcely respond to high frequency input though they can response to low frequency input, their response falls in a range of high frequency. As a result their frequency vs. response characteristic deviates from the preferable curve B of flatness shown in FIG. 2 to show the curve A. When the frequency vs. response characteristic is not flat, even if an optical pulse signal of rectangular waveform shown in FIG. 3A is inputted into the light detecting device, its output waveform comes to be distorted as shown in FIG. 3B. Therefore, since it is impossible to faithfully monitor output of the semiconductor laser, it is difficult to control the semiconductor laser in real time.
If a distance L from the side face of the light detecting device to its active area (p.sup.+ -type diffusion laser 104) is made short in order to decrease the number of slow-response carriers to be generated, electrons and holes generated in the outside of a depletion layer can reach the depletion layer in a shorter time, and as a result the response of the device can be made faster, so flatness of the frequency vs. response characteristic is improved.
There is a problem, however, that dark current is increased if the distance L between the active area and the side face of the light detecting device is shortened. A result of measurement of the dark current characteristic by the inventors is described later.
An object of the present invention is to provide a light detecting device for monitoring which is better in frequency vs. response characteristic and less in dark current.
The light detecting device of the present invention comprises a first semiconductor layer containing a first conductive impurity, a second semiconductor layer containing a first conductive impurity which is lower in density than the first semiconductor layer and is formed on the first semiconductor, a third semiconductor layer containing a second conductive impurity which is formed in the upper part of the second semiconductor layer in the active area, and a fourth semiconductor layer containing a second conductive impurity which is formed around the third semiconductor layer with an interval between the third semiconductor layer and the fourth semiconductor layer. The first conductive impurity is one of n-type and p type impurities, and the second conductive impurity is the other of them.
When the second semiconductor layer of the light detecting device is irradiated with light, pairs of electrons and holes are formed in the second semiconductor layer, and they are moved by a diffusion potential and a reverse bias voltage. Electrons or holes generated in the outside of the depletion layer around the third semiconductor layer reach the third semiconductor layer at a slow speed, but in the case that the semiconductor layer outside the third semiconductor layer is small in area, the number of carriers is small, so the frequency vs. response characteristic of output of the light detecting device becomes flat.
The light detecting device of the present invention remarkably reduces the dark current in comparison with a light detecting device having no fourth semiconductor layer. The reason for this is as follows.
There are many nuclei of generation and recombination on the end face of a compound semiconductor layer of a light detecting device. Since carriers generated here flow into the depletion layer of pn junction through a channel formed in a hetero interface of the semiconductor layer, dark current is generated. When a barrier of pn junction is formed in the middle of the channel as in this invention, it is difficult for the carriers to move to the depletion layer because of obstruction of the channel and thus the dark current is reduced.
The longer the distance for the carriers to diffuse is, the slower the response of the output current is. Therefore, the shorter the distance between the third and fourth semiconductor layers are, the better the response is. The smaller a ratio of the number of carriers generated outside the depletion layer to the number of carriers generated in the depletion layer around the third semiconductor layer is, the better the response in high frequency is improved.
Carriers generated outside the depletion layer is reduced by making thinner the semiconductor layer in the outside of the third semiconductor layer. Reflectivity of the surface of the semiconductor layer in the outside of the third semiconductor layer is improved by controlling thickness of the protection film of the surface of the semiconductor layer. Thus, by carriers generated outside the depletion layer are reduced.